1. Field of the Invention
The present invention relates to an exhaust control valve assembly of a two stroke internal combustion engine. More particularly, the invention relates to a technically efficient exhaust control assembly wherein both the main exhaust control valve and the at least one auxiliary exhaust control valve can be controlled by only one actuator.
2. Discussion of the Prior Art
In order to ensure that two-cycle internal combustion engines can achieve a high peak power output at high engine speeds, care must be taken to ensure that the cylinder is well charged and that there are the smallest possible charge losses in the upper part of the engine-speed range; this can be achieved by exploiting resonance effects, i.e., by appropriately early opening of the exhaust passage. However, because of the comparatively great vertical dimension of the cross section of the exhaust outlet, in the mid and lower engine-speed ranges this matching of the exhaust system to the peak power output of the internal combustion engine causes not only a noticeable loss of usable capacity but also a marked increase in charge losses. As a result, torque is reduced and specific fuel consumption increases. An increase in torque with simultaneously reduced fuel consumption can be achieved in the lower engine speed ranges only by having the exhaust outlet open for a shorter time. In this connection, it is known, for example from AT 380 537 B, that a control slide valve can be fitted in the exhaust passage. This control slide valve is supported so as to slide in a through guide that is radial relative to the cylinder, in the side wall of the exhaust passage that is proximate to the cylinder head. At its free face end, this control slide valve has a control edge that is matched to the cylinder bore so that, when the control valve is in its working position, in which it is advanced against the cylinder, the vertical dimension of the exhaust outlet cross-section is partially covered. This results in a corresponding reduction of the exhaust period because of the delayed opening and earlier closing of the exhaust passage, and thus to improved charging of the cylinder in the middle and lower speed ranges.
If, in addition to a main exhaust outlet, secondary side exhaust outlets that are connected through channels to the exhaust passage that is connected to the main exhaust outlet are provided in the cylinder in order to improve exhaust conditions at the beginning and at the end of the exhaust gas discharge, then these secondary exhaust outlets are blocked off, for example by cylindrical shutoff bodies, in order to reduce the exhaust period. These shutoff bodies pass through the side passages, and either can rotate or slide between a closed position and an open position, thereby closing or opening the secondary exhaust outlets. The main control exhaust valve that is provided in the main exhaust passage and which is designed as a control flap, is so supported in bearings such that it can rotate or slide in the area of the side wall of the exhaust passage that is proximate to the cylinder head, and so that it can also be rotated or slid partially into the exhaust passage as to reduce the effective vertical dimension of the main exhaust outlet. In order that the cylindrical shutoff bodies, which incorporate a recess for releasing the side channels, can be actuated as a function of control flap, the control flap—whose axis of pivot extends parallel to a tangent to the cylinder—is connected through a sliding-block guide to the cylindrical shutoff bodies. One important disadvantage of this known exhaust period control are the considerable design costs that are occasioned, on the one hand, by the cylindrical shutoff bodies in the side channels, which must be supported in bearings so as to be rotatable and, on the other hand, by the necessary drive connection between these shutoff bodies and the control flap valve, the axis of pivot of which is offset by an angle of 90° relative to the axes of rotation of the shutoff bodies.
Finally, EP 0 141 650 A2 describes how the side secondary exhaust outlet, which is adjacent to the main exhaust outlet, is controlled by means of a shutoff slide as a function of engine speed, so that the secondary exhaust outlet is first opened at higher speeds. However, this known design incorporates no control body for controlling the exhaust period of the main exhaust outlet.
JP 7-279675 A describes a system for controlling the open position of the secondary exhaust outlet by means of rack drives, as a function of the open position of the control slide valve for a main exhaust outlet. This system is costly from the standpoint of construction and requires a great deal of space.
In order to keep the gap between the slide valve and the piston as small as possible along the whole of the positioning travel when there is an angle between the slide-valve guide and the axis of the cylinder, and to ensure that when it is in its lower, extended, position, the slide valve rests snugly against the radius of the cylinder wall across the complete width of the exhaust outlet slot, it is known from DE 195 35 069 A1 that the control slide valve can be provided with a guide surface that is curved in the direction of the cylinder axis, this guide surface being a part of a curve that is perpendicular to the direction of movement of the slide valve.
It is known from prior art publication AT399204B to design the main exhaust control valve as a two-part sliding valve. By this design two different settings of the port timing can be established.
It is further known in the prior art to provide a exhaust control valve having a main exhaust valve and integrally formed auxiliary exhaust valves. Due to the big dimension of such a exhaust control valve such a design is especially susceptible to jamming following thermal expansion of the components.
It is further known from the prior art publication AT407555B to design a main and secondary exhaust control valves such that the main and the secondary exhaust control valves can be actuated by a single actuator. The main exhaust control valve is biased by a spring towards its extended position and can be moved to its retracted position through a coupling by the movement of the secondary exhaust valves. The secondary exhaust valves are coupled by a yoke which connects the secondary exhaust valves to the actuator. In trials this design has resulted in difficulties concerning the fit of the components of the exhaust control device in the corresponding guides in the cylinder block.
Thus there is a need for an improved exhaust valve assembly for a two-stroke internal combustion engine.